Gastrointestinal Motility and Smooth Muscle

Early in life, children notice that strange gurgling sounds sporadically emanate from their "stomachs," particularly in periods between meals. This simple observation reflects the fact that the digestive tube is quite muscular and that muscle contractions and motility are integral parts of digestive function. It follows that derangements in gastrointestinal motility can cause or result from digestive tract disease and that drugs which alter gastrointestinal motility affect digestive function.

Two fundamental patterns of motility are conducted by the digestive tube:

Propulsion: foodstuffs must be propelled along the length of the digestive tube in order to be subjected to the sequential series of processing involved in dissassembly and absorption.

The principal type of propulsive motility, seen particularly in the esophagus and small intestine, is peristalsis - a ring of muscle contraction appears on the oral side of a bolus of ingesta and moves toward the anus, propelling the contents of the lumen in that direction; as the ring moves, the muscle on the other side of the distended area relaxes, facilitating smooth passage of the bolus.

Peristalsis

Mixing: If ingested materials were simply propelled through the digestive tube, you would expect very poor digestion and absorption, because the digestive enzymes would not be adequately mixed with the ingesta and the bulk of the ingesta would not come in contact with the epithelial cells that absorb nutrients.

Segmentation contractions are a common type of mixing motility seen especially in the small intestine - segmental rings of contraction chop and mix the ingesta. Alternating contraction and relaxation of the longitudinal muscle in the wall of the gut also provides effective mixing of its contents.

Segmentation Contractions

Except for the first section of the esophagus, all the the muscle in the wall of the digestive tube is smooth muscle. Indeed, the patterns of motility seen in the gut are characteristic of smooth muscle, which has properties distinctly different from skeletal muscle.

Smooth muscle fibers are arranged in intertwined, rather indistinct bundles, aligned in most areas of the tube in circular and longitutinal layers. Individual smooth muscle fibers are connected to neighboring smooth muscle cells by gap junctions, which allow these cells to be electrically coupled. The important consequence of this electrical coupling is that when an area of smooth muscle becomes depolarized, that depolarization spreads outward through adjacent sections of smooth muscle - this results in a well-coordinated contraction of, for example, an entire ring of circular smooth muscle. Without electrical coupling through gap junctions, one would imagine that you would see contraction only of patches of circular or longitudinal muscle, which would have little effect on propulsion or mixing of ingesta.